1. Field of the Invention
The present invention relates to a device for monitoring a polishing pad and a method thereof. More particularly, it relates to an apparatus and a method for measuring the polishing pad to determine the need for replacement.
2. Description of the Related Art
Chemical mechanical polishing (CMP) process is a method for flattening a surface of a substrate by polishing pad and abrasive slurry.
The substrate is set on a rotatable carrier head and is contacted by a rotating polishing pad having a coarse surface.
The abrasive slurry is sprayed on the polishing pad, preceding a chemical or mechanical reaction with the substrate.
The surface of the portion polishing pad becomes glazed after a certain period of use. The glazed surface of the polishing pad can damage the wafer, so the polishing pad is conditioned to deglaze the surface thereof. The polishing pad is exchanged after a certain number of polishings, such as 300–500.
A conventional chemical mechanical polishing machine is shown in FIG. 1a. The chemical mechanical polishing machine comprises a polishing spindle 101, a platen 102, a slurry supply unit 103, a polishing carrier 104, a polishing pad 105, all acting on a wafer 106. The polishing carrier 104 is disposed on the polishing spindle 101. The polishing pad 105 is disposed on the platen 102. The wafer 106 is held by the polishing carrier 104.
After a certain period of time of polishing or a certain number of polishings, the pad is conditioned to restore its uniformity, or, if beyond repair, replaced.
There is currently no way to quickly determine if replacement of the polishing pad is required. The polishing pad 105 as shown in FIG. 1a is removed from the platen and viscose (not shown) is removed therefrom. Thickness of the polishing pad from the edge to the center of the polishing pad is measured, generating a thickness profile as shown in FIG. 1b. The polishing pad is replaced when the thickness of the polishing pad is not uniform as shown in FIG. 1b. 
A conventional polishing pad 201 is shown in FIGS. 2a–2b. The polishing pad 201 is hollow, and gaps 202 and 203 form on the inner wall and outer wall of the polishing pad 201 respectively. The thickness of the polishing pad 201 is measured by the gaps 202 and 203, and the thickness profile is obtained as shown in FIG. 2c. The method of measuring the thickness of the polishing pad by the gaps 202 and 203 as above uses less time but the resulting measurement is incomplete.
Another conventional chemical mechanical polishing machine is shown in FIG. 3. The chemical mechanical polishing machine comprises a body 302, a polishing spindle 303, and a carrier 304. The polishing pad 301 is grasped by the carrier 304. The polishing pad 301 comprises an upper pad 301a and a lower pad 301b. A wafer (not shown) is situated between the upper pad 301a and the lower pad 301b. The polishing pad 301 is situated in a predetermined position in the chemical mechanical polishing machine. The polishing pad 301 is grasped by the carrier 304 before the wafer is polished.
The polishing spindle 303 moves up and down corresponding to the rough surface of the polishing pad 301. The operator must estimate the thickness of the polishing pad 301 after a period of use. The polishing pad 301 is replaced if the operator determines the thickness of the polishing pad is insufficient. The pad is used if the operator determines the thickness of the polishing pad 301 is sufficient.
Replacement of the polishing pad is decided by the operator of the chemical mechanical polishing machine, and may be performed needlessly if the thickness of the polishing pad is sufficient but judged incorrectly by the operator. Conversely, wafers may be abraded if the thickness of the polishing pad is insufficient.